BE: Support self-signed certs for OIDC

[Haarolean]

Currently to achieve this one has to do mental gymnastics like this:

JAVA_OPTS: >-
        -Djavax.net.ssl.trustStore=/jmx/clienttruststore
        -Djavax.net.ssl.trustStorePassword=12345678

This happened due to:

1. unifying truststore/keystore usage per cluster
2. with 1) having no universal truststore/keystore applicable to non-clustered things (authentication, etc)

TODO:

1. Implement an app-wide truststore/keystore support
2. Use it for accessing:

• OIDC endpoints
• ?

[Haarolean]

Seems to be solvable via these config props:

server:
  ssl:
    trust-store: classpath:keycloak-truststore.jks
    trust-store-password: changeit

Documented here, closing.